Optical transmission systems are used in a variety of applications. Some applications transmit data in an optical signal carried on an optical fiber. Other applications transmit an optical signal through free space to an optical receiver. Examples of such systems are defined in the Infrared Data Association (“IrDA”) communications standard. The IrDA communications standard is used when designing infrared (“IR”) data ports on electronic devices, such as computers, personal digital assistants, and mobile telephones.
IR data transmissions between devices using the IrDA communications standard transmit information at communication speeds typically between about 100 kbps and 16 Mbps. The IrDA specification supports optical communications links between two nodes (electronic devices) from about 0 meters to about 1 meter apart, and it is desirable for IR receivers to detect low light pulse levels in order to accurately receive data from IR transmitters that are either far away or are weakly emitting IR light.
An IR receiver typically has a photodetector, such as a “PIN” diode, converting optical energy from the IR pulse into an electrical current, which is converted into a voltage data signal. The voltage data signal is amplified by one or more gain stages that generally have sufficient gain to detect low-level signals.
FIG. 1 is a prior art block diagram of a receiver circuit 100. A PIN diode 102 is modeled as a current source 105 in parallel with a capacitor 107 and a shunt resistor 106 because PIN diodes used as photodetectors are reverse biased, forming a depletion junction across the diode which is capacitive. Normally, the shunt resistor 106 is very large and can be normally ignored. When light, represented by an arrow 104, shines on the PIN diode 102, it produces a current, which is modeled by a constant current source 105 in parallel with a resistor 109, that flows through a load resistor 106, producing a voltage at node 108. The voltage at node 108 is provided to one input of a differential amplifier 110. The receiver circuit 100 includes additional amplifiers 112, 114, and a comparator 116, which compares the single-ended output 118 of amplifier 114 to a threshold (reference) voltage VTH. The amplifier 114 is used to convert the differential signal into single-ended signal at the node 118. The comparator 116 provides an output 120 when the single-ended output exceeds VTH, which is provided to logic 122 of the receiver 100.
Both the PIN diode 102 and a “dummy” capacitor 124 are biased by a voltage supply 126. The dummy capacitor 124 has a capacitance similar to the reverse-biased capacitance of the PIN diode 102. If noise is present on the voltage supply, for example, the dummy capacitor produces a current through a dummy load resistor 128 (which typically has the same resistance as load resistor 106) similar to the noise current produced by the photodetector 102 and load resistor 106. Noise from the voltage supply 126 is expressed as a common mode voltage at both inputs 130, 132 of the differential amplifier 110, and is rejected due to good common mode rejection ratio of the differential amplifier 110. Thus, the rejected common input signal does not trigger a false data event.
Unfortunately, process variations in the fabrication of PIN diodes, variations in circuit components in the voltage supply and bias circuitry, biasing point of the PIN diode, variations in the dummy capacitor, and other factors can result in an assembled receiver circuit 100 with a poor common mode rejection ratio (“CMRR”). Therefore, optical receiver circuits with improved CMRR are desirable.